This invention relates to the blocking of lens blanks, and more particularly to the precision attachment of blocks to lens blanks for their conversion into finished lenses.
In the finishing of lens blanks for eye glasses, it is necessary to translate a prescription provided by an oculist or optometrist into lenses which are suitable and appropriate for mounting into a frame that meets the physical requirements of the user.
The prescription specifies the various powers of the users lenses, and their optical axes, along with the users pupilary distance (P.D.) and the measurements of the frame that has been selected. A pupilary distance is the measure of the separation between the pupils of the user. It is apparent that the pupilary distance varies from one user to another, and that it must be coordinated with the measurements of the selected frame.
In translating a lens prescription into a finished product, an optical technician begins by selecting a lens blank in accordance with the user's optical correction. In the general case where the lens has a toric, i.e. toroidal, outer surface and a spherical inner surface, the lens has an optical center and prescribed optical axis in accordance with the required cylindrication of the lens. In the special case where the lens has only spherical surfaces, it has only an optical center.
The optical center and axis of a lens blank are located using an instrument known as a lensometer. The blank is positioned in the viewer of the lensometer and a reticle is focused on it. A typical lensometer reticle has a triplet of parallel lines and an orthogonal bisecting line. When the bisecting line is focused on the lens blank, its spherical power is indicated by the focusing control. The lens blank is then centered horizontally.
In the next step, the triplet is focused on the lens and the focusing control provides an indication of the cylindrical power (which is given by the difference between the indicated value and the spherical reading). The lens blank is then centered vertically.
In the final step, the reticle is rotated until the bisector appears as a solid, unfragmented line. The amount of rotation indicates the inclination of the optical axis and the intersection of the bisector and the central line of the triplet gives the optical center.
The optical center and two points on the optical axis are then marked using an inking lever mechanism appended to the lensometer.
Once marked the lens blank is removed from the lensometer and a "cutting" line drawn on it through the three points to indicate the optical axis. In the next step a special scale is placed along the optical axis to locate the "finishing center". The latter is at the position by which the optical center is de-centered to accommodate the lens to the frame of the user. Thus if the P.D. is 68 millimeters and the frames have a bridge size, nose span, of 14 millimeters and a lens size of 48 millimeters making a total frame span of 62 millimeters, the optical center of each lens blank must be de-positioned by three millimeters (one-half of the difference between the frame span and the P.D.)
A temporary mounting known as a "block" is then temporarily affixed to the lens at the finishing center. The block is in the form of an alloy with a pattern that allows the lens blank to be rotated with respect to a finishing wheel.
It is apparent that if the optical axis is not properly located, or if the finishing center is not properly located, the finished lens will be in error.
In addition, the manual techniques in present use require operators with considerable skill, and are time consuming.
Even if the operator is highly skilled, he is confronted wit play and inexactness in the instruments that he uses.
The result is that an estimated 30 percent of the lenses are incorrectly blocked, of which about fifteen percent are so inaccurate that they must be discarded. While the remaining fifteen percent are not rejected, they are nonetheless less accurate than they should be.
Accordingly, it is an object of the invention to increase the precision with which lens blanks can be blocked. A related object is to eliminate the need for the marking of lens blanks. Another related object is to eliminate the need for special scales in the de-centering of lens blanks.
Another object is to reduce the number of rejects encountered in the blocking of lenses. A related object is to raise the level of accuracy of lens blanks that are accepted.
A further object is achieve the precision blocking lenses with only semi-skilled operators.